The present invention generally relates to optical disk drives for recording and/or reproducing information signals on and from an optical disk, and more particularly to an optical disk drive having a separated optical system that includes a stationary optical system and a movable optical system for focusing the optical beam on the recording surface of the optical disk.
In the optical disk drives for recording and/or reproducing information signals on and from an optical disk or in the optical disk drives used explicitly for reproducing information signals from an optical disk, it is essential to minimize the access speed. In order to achieve this object, conventional optical disk drives have been constructed to have a separated construction for the optical system such that the optical system includes a stationary part fixed upon a body of the disk drive and a movable part movable with respect to the disk drive body. Thereby, the tracking and focusing control of the optical beam is achieved by moving the movable part alone with respect to the optical disk that is mounted on the optical disk drive. As the movable part is constructed to have a minimum inertia, one can maximize the access speed when reading or writing optical information.
In the optical disk drives having such a separated construction for the optical system, however, there arises a problem in that the optical path length increases substantially, and such an increase in the optical path length tends to case a problem of beam offset between the optical beam that exists from the stationary part toward the optical disk and the optical beam that returns from the optical disk after reflection. Such an offset in the optical beam causes an unwanted tracking error and the optical disk drives having the foregoing construction tend to suffer from the problem of unstable tracking control. Thus, there is a demand for an optical system that has the separated construction and simultaneously the problem of beam offset is minimized.
FIG. 1 shows an example of the conventional optical disk drive.
Referring to FIG. 1, the optical disk drive 1 includes an optical disk 2 mounted thereon for rotation by a motor 3, wherein the optical disk 2 carries thereon a number of concentric or spiral tracks not illustrated. The optical disk 2 includes a data region 4 defined by an outermost periphery 5 and an innermost periphery 6, for recording information. Further, the optical disk drive 1 includes an optical system 10 that in turn consists of a stationary part 11 and a movable part 12, wherein the stationary part 11 is mounted on a base body 9 of the optical disk drive 1.
As usual, the stationary part 11 includes a laser diode 13 for producing an optical beam, a collimator lens 65 for converting the optical beam produced by the laser diode 13 to form a parallel optical beam, a beam splitter 14 for reflecting the optical beam that has been reflected back from the optical disk 2, a two-field photodiode 15 disposed to receive the optical beam from the beam splitter 14 for detection thereof, and a mirror 18 disposed to receive the optical beam passed through the beam splitter for reflecting the same toward the movable part 12. Further, the mirror 18 is driven by a tracking mechanism 19 such that the mirror 18 is tilted by a minute angle about an axis 20 for achieving an optical tracking of the optical beam. It should be noted that the axis 20 is formed substantially coincident to the mirror plane of the mirror 18.
The output of the two-field photodiode 15 is supplied to a differential amplifier 21, and the differential amplifier 21 produces an output signal as a tracking error signal. The tracking error signal thus produced is outputted from an output terminal 22.
The movable part 12, in turn, is provided in correspondence to the data region 4 and is held movable on the base body 9 in a radial direction of the optical disk 2 as represented by an arrow 30. The movable part 12 carries thereon an objective lens 32 for focusing the optical beam on the optical disk 2 to form a tiny optical spot thereon, wherein the objective lens 32 is carried by a focusing mechanism 33 that moves the lens 32 in the direction perpendicular to the surface of the disk 2 for focusing control. Further, there is provided a mirror 34 that reflects the optical beam supplied from the stationary part 11 toward the optical disk 2.
In operation, the tracking mechanism 19 is controlled upon a control signal supplied thereto such that the mirror 18 is tilted about the axis 20, and the optical spot 31 moves in the radial direction of the disk 2. There, it should be noted that the optical beam 35 that returns from the movable part 12 to the stationary part 11 coincides with the optical beam 17 that goes from the stationary part 11 to the movable part 12, as long as the mirror 18 is not tilted. Thereby, there is no beam offset and an optical spot 36 is formed on the center of the two-field photodiode 15. On the other hand, when the beam spot 36 offsets from the center, the optical intensity distribution changes and the differential amplifier 21 produces a tracking error signal.
FIG. 2 shows a relationship between the actual tracking error on the optical disk 2 and the tracking error signal produced by the differential amplifier 21. Thus, it will be noted that there is no substantial tracking error signal as long as the optical spot 31 on the disk 2 is located at the center of track defined on the optical disk 2.
When a tracking control signal is supplied for the purpose of causing the optical spot to move in correspondence to the disk eccentricity .epsilon. as indicated in FIG. 3, the drive mechanism 19 is activated and the mirror 18 is tilted by a minute angle .theta.. In response to this, the spot 31 on the optical disk 2 moves to another track separated by a distance corresponding to the eccentricity .epsilon.. When such a tracking control is achieved properly, it should be noted that the optical spot 36 on the two-filed photodetector 15 should remain at the center of the field. However, this is not the case because of the reason described below.
When the mirror 18 is tilted by a minute angle .theta. the returning optical beam 35a shifts with respect to the exiting optical beam 17a by an offset D as indicated in FIG. 3, and such an offset D in turn causes an offset of the beam spot 36 in the upward direction by a distance .beta. on the two-field photodetector 15. Thereby, an offset voltage V.sub.1 is produced inevitably by the differential amplifier 21 indicating an erroneous tracking error 37 as represented in FIG. 2. There, the magnitude of the offset voltage V.sub.1 is generally proportional to the magnitude of the beam offset D, and the beam offset D is given by the relationship EQU D=2(L+b) tan 2.theta., (1)
where L represents the distance between the mirror 18 and the mirror 34, while b represents the distance between the mirror 34 and a back focal point 38 of the objective lens 32. It should be noted that the back focal point 38 is located close to the mirror 34.
FIGS. 4 and 5 show other situations wherein the beam offset occurs respectively for the case where the movable part 12 is located close to the outermost periphery 5 of the data region 4 and where the movable part 12 is located close to the innermost periphery 6. Similarly to the previous case, the beam offset occurs in response to the tilting of the mirror 18 by a minute angle .theta..
In view of the foregoing Eq. (1) and the geometry shown in FIGS. 4 and 5, one derives the relationship between the beam offset D and the position of the movable part 12 as represented in FIG. 6 by a line II. As shown therein, the magnitude of the beam offset D under a constant tilt angle .theta. of the mirror 18 increases with increasing distance of the movable part 12 from the outermost periphery 5 of the data region 4. In correspondence to the beam offset D, there appears a relationship between the offset voltage V.sub.1 and the position of the movable part 12 as indicated in FIG. 7 by a line III. Again, it will be noted that the offset voltage V.sub.1 increases with increasing distance of the movable part 12 from the outermost periphery 5 toward the innermost periphery 6. Obviously, such an erroneous tracking error signal causes an unstable operation of the tracking system of the optical disk device.